Embryo of Dicot Seeds

Our Objective

Our objective is to identify the different parts of an embryo of dicot seeds - pea, gram and bean.

The Theory

A seed is a small embryonic plant enclosed inside a seed coat. It is the ripened and fertilised ovule of gymnospermic and angiospermic plants. A seed has a three parts:

  1. An embryo
  2. A supply of nutrients for the embryo
  3. A seed coat

The embryo is an undeveloped plant inside a seed from which a new plant develops. All seeds do not have the same size, shape and colour. We will be looking at the embryo of a pea, gram and bean seed to identify their different parts.

Plant Embryo

Plant embryos in seeds have structures called cotyledons. A cotyledon is the central portion of a seed embryo to which the epicotyls- the immature shoot, and the radicle- the immature roots, are attached.

Plants are classified according to the number of cotyledons present in the embryo. If the embryo has one cotyledon (monocotyledon) it is a monocot plant, and if there are two cotyledons (dicotyledon), it is a dicot plant.

Seed Subdivision

Based on the type and location of storage materials, seeds can be subdivided into endosperms and non endosperms.

Endospermic Seeds

Endospermic seeds are those that have an endosperm in the mature seed. It is fleshy, oily, surrounds the embryo, and functions as the sole food storage organ. Inside the seed coat, a thin and papery cotyledon is present. Monocot plants have endospermic seeds.

Non-endospermic Seeds

Non-endospermic seeds do not have an endosperm in the mature seed. The cotyledons are thick and fleshy, and function as the sole food storage organs. Dicot plants have non-endospermic seeds.


Seed Germination

Germination is the process in which the seed breaks apart, and the embryo inside begins to grow with the help of water and nutrients from the soil. Seeds normally begin to germinate when they get an appropriate range of soil, temperature and when water and oxygen are available. During germination the seeds absorb water either through the micropyle (pore like opening), if it is present, or through the testa (outer seed coat) when it is permeable. The entry of water activates the embryo cells.

Germination in a Dicot Seed (Pea, Gram and Bean)

The plumule, radicle, and hypocotyl are the three components that make up the embryonic axis. The term "hypocotyl" (which means "below the cotyledons") refers to the area of the embryo that lies between the cotyledon attachment point and the radicle. The embryonic axis ends in a radicle, also known as the embryonic root, from which the root will eventually emerge. In dicots, the hypocotyls grow above the soil to form the plant's stem. Because monocots do not display stem elongation, the hypocotyl is not visible above ground in monocots. The epicotyl is the portion of the embryonic axis that extends above the cotyledons. The epicotyl, juvenile leaves, and the shoot apical meristem make up the plumule. 

When dicot seeds germinate, the epicotyl takes the form of a hook with the plumule pointing down. The plumule hook is the name for this morphology, which endures if germination takes place in the dark. The plumule is thus guarded against harm while the epicotyl pushes through the hard, abrasive soil. When exposed to light, the epicotyl keeps growing longer, the leaves of the young foliage turn towards the sun and enlarge, and the hypocotyl hook straightens up. The radicle is developing and creating the major root currently as well. The normal dicot tap root system is created as the lateral root system branches off to all sides as it grows downward to form the tap root. 

The pea, gram and bean seeds have two seed coats that envelope and protect the embryo. The outer seed coat called the testa is thick and strong. The inner coat is called tegmen, which is thin and membranous.

The outer seed coat has a notch on one side and a scar along the notch, called the hilum. It is the hilum that attaches the seed to the inner margin or the seed stalk. There is a small pore called micropyle located at one end of the hilum. The seed imbibes water through the microphyle.

Once soaked for the required time, the part of the plant that first emerges from the seed is the embryonic root known as the radicle. The radicle allows the seedling to anchor itself to the ground and start absorbing water.

After the root starts absorbing water, an embryonic shoot emerges from the seed. This shoot comprises of three main parts- the cotyledons (seed leaves), the hypocotyl (section of shoot below the cotyledons), and the epicotyl (section of shoot above the cotyledons).


Learning Outcomes

  • Students understand the meaning of cotyledons, endosperm, embryo, etc.
  • Students understand the different parts of the embryo of the dicot seed.
  • Students understand the difference between endospermic and non-endospermic seeds.